Thermochemical analysis and kinetics aspects for a chemical model for camphene ozonolysis

The ozonolysis of cyclic monoterpenes: a computational review

Monoterpenes are prevalent organic compounds emitted to the atmosphere, via biogenic activities in various types of plants. Monoterpenes undergo atmospheric decomposition reactions derived by the potent atmospheric oxidizing agents, OH, O3, and NOx. This review critically surveys literature pertinent to the atmospheric removal of monoterpenes by ozone. In general, the ozonolysis reactions of monoterpenes occur through the so-called Criegee mechanism. These classes of reactions generate a wide array of chemical organic and inorganic low vapor pressure (LVP) species. Carbonyl oxides, commonly known as Criegee intermediates (CIs), are the main intermediates from the gas-phase ozonolysis reaction. Herein, we present mechanistic pathways, reactions rate constants, product profiles, thermodynamic, and kinetic results dictating the ozonolysis reactions of selected monoterpenes (namely carene, camphene, limonene, α-pinene, β-pinene, and sabinene). Furthermore, the unimolecular (vinyl hydroperoxide and ester channels) and bimolecular reactions (cycloaddition, insertion, and radical recombination) of the resulting CIs are fully discussed. The orientations and conformations of the resulting primary ozonides (POZs) and CIs of monoterpenes are classified to reveal their plausible effects on reported thermokinetic parameters.

Hydration and Secondary Ozonide of the Criegee Intermediate of Sabinene

A computational study of the formation of secondary ozonide (SOZ) from the Criegee intermediates (CIs) of sabinene, including hydration reactions with H₂O and 2H₂O, was performed. All of the geometries were optimized at the B3LYP and M06-2X with several basis sets. Further single-point energy calculation at the CCSD(T) was performed. Two major pathways of SOZ formation suggest that it is mainly formed from the sabinene CI and formaldehyde rather than sabina ketone and formaldehyde-oxide. However, in both pathways, the activation energies are within a range of ±5 kJ mol^-1. Furthermore, the hydration reactions of the anti-CI with H₂O and 2H₂O showed that the role of the second water molecule is a mediator (catalyst) in this reaction. The dimer hydration reaction has lower activation energies than the monomer by 60 and 69 kJ mol^-1, at the M06-2X/6-31G(d) and CCSD(T)+CF levels of the theory, respectively. A novel water-mediated vinyl hydroperoxide (VHP) channel from both the monomer and dimer has been investigated. The results indicate that the direct nonmediated VHP formation and dissociation is interestingly more possible than the water-mediated VHP. The density functional theory calculations show that the monomer is faster than the dimer by roughly 22 kJ mol^-1. Further, the infrared spectrum of sabina ketone was calculated at B3LYP/6-311+G(2d,p); the calculated carbonyl stretching of 1727 cm^-1 is in agreement with the experimental range of 1700-1800 cm^-1.

OH- and O3-initiated atmospheric degradation of camphene: temperature dependent rate coefficients, product yields and mechanisms

Gas-phase rate coefficients for the reactions of OH and O₃ with camphene have been measured over the temperature range 288–311 K using the relative rate method.

Computational investigation into the gas-phase ozonolysis of the conjugated monoterpene α-phellandrene

Reaction with ozone is a major atmospheric sink for α-phellandrene, a monoterpene found in both indoor and outdoor environments, however experimental literature concerning the reaction is scarce. In this study, high-level G4(MP2) quantum chemical calculations are used to theoretically characterise the reaction of ozone with both double bonds in α-phellandrene for the first time. Results show that addition of ozone to the least substituted double bond in the conjugated system is preferred. Following addition, thermal and chemically activated unimolecular reactions, including the so-called hydroperoxide and ester or 'hot' acid channels, and internal cyclisation reactions, are characterised to major first generation products. Conjugation present in α-phellandrene allows two favourable Criegee intermediate reaction pathways to proceed that have not previously been considered in the literature; namely a 1,6-allyl resonance stabilised hydrogen shift and intramolecular dioxirane isomerisation to an epoxide. These channels are expected to play an important role alongside conventional routes in the ozonolysis of a-phellandrene. Computational characterisation of the potential energy surface thus provides insight into this previously unstudied system, and will aid future mechanism development and experimental interpretation involving α-phellandrene and structurally similar species, to which the results are expected to extend.

Ozonolysis reactions of monoterpenes: a variational transition state investigation

The O3-initiated oxidation reactions of α-pinene ([1S,5S]-2,6,6-trimethylbicyclo[3.1.1]hept-2-ene), β-pinene ([1R,5R]-6,6-dimethyl-2-methylenebicyclo[3.1.1]heptane), camphene ([1R,4S]-2,2-dimethyl-3-methylenebicyclo[2.2.1]heptane) and sabinene ([1R,5R]-4-methylene-1-(1-methylethyl)bicycle[3.1.0]hexane), four monoterpenes typically emitted into the atmosphere, were studied at the B3LYP/6-31+G(2d,2p) level of theory. The rate coefficients were calculated on the basis of the variational transition state theory for two kinetic models, in order to investigate the reaction mechanism: first assuming a direct bimolecular reaction and the second, assuming the formation of a prebarrier-complex, which further reacts forming the corresponding molozonide. The barrier heights leading to the formation of exo-conformers of the molozonides of α-pinene and camphene are lower than the barrier heights for the formation of the endo-conformers of these molozonides, whereas the inverse trend is observed for β-pinene and sabinene. The canonical variational rate coefficients are found in reasonable agreement with the experimental data, especially when the prebarrier complexes are considered. Microcanonical variational rate coefficients are also calculated, as a final validation test, being found in an expected agreement with the canonical rate coefficients. The best predictions for the rate coefficients at 298 K, based on the microcanonical variational method, for α-pinene, β-pinene, camphene, and sabine are (in units cm(3) molecule(-1) s(-1)): 6.92 × 10(-17), 1.06 × 10(-17), 4.61 × 10(-19), and 4.81 × 10(-17), respectively. Our results suggest that the prebarrier complex is an important specie in the ozone addition mechanism and should be taken into account for the proper description of the overall kinetics.

Theoretical study of Δ-3-(+)-carene oxidation

In this work, the rate-limiting steps of Δ(3)-carene oxidation by ozone and OH radicals were studied. The thermochemical and kinetic parameters were evaluated using the B3LYP, PBE1PBE and BHandHLYP functionals, coupled cluster methods and the 6-311G(d,p) and 6-311++G(d,p) basis sets. The attack on the double bond may occur in different orientations, leading to different oxidation products. The rate coefficients of each step of the reactions were evaluated using conventional canonical transition-state theory and variational canonical transition-state theory whenever necessary. The theoretical rate coefficient for the ozonolysis mechanism, evaluated at the CCSD(T)/6-31G(d,p)//PBE1PBE/6-311++G(d,p) level, was 2.08 × 10(-17) cm(3) molecule(-1) s(-1). The coefficient for the oxidation initialised by the OH radical, calculated at the BHandHLYP/6-311++G(d,p) level, was 5.06 × 10(-12) cm(3) molecule(-1) s(-1). These values are in reasonable agreement with the experimental results. The importance of these reactions in atmospheric chemistry is discussed.